Internal fault indicator for power electrical devices

ABSTRACT

An internal fault indicator for an electrical device is triggered by a sudden increase in pressure as occurs when an insulation failure creates an electric arc. The heat released in the arc is transferred onto the surrounding volume causing localized overheating, vaporization and decomposition of the insulating material. The resulting pressure surge moves a diaphragm. The movement of the diaphragm releases a spring driven plunger from a barrel which extends through the housing of the electrical device. Prior to activation the plunger is held in an “armed” position by a retaining pin. Upon triggering, the plunger is pushed by the spring until it protrudes from the housing to provide a visual signal of the internal fault. A pressure relief valve may be integrated with the internal fault indicator.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of PCT international application no.PCT/CA01/00873 filed Jun. 14, 2001 entitled INTERNAL FAULT INDICATOR FORPOWER ELECTRICAL DEVICES which designates the United States and is acontinuation-in-part of U.S. patent application Ser. No. 09/593,749filed Jun. 14, 2000, now U.S. Pat. No. 6,429,662, both of which arehereby incorporated herein by reference.

TECHNICAL FIELD

[0002] This invention relates to indicators for signalling theoccurrence of internal faults in oil-filled or gas-filled electricalequipment, such as transformers, reactors, capacitors and the like. Theinvention has particular application to electrical components used inelectrical power distribution systems. In particular, the inventionrelates to internal fault indicators which display a visible indicatorwhen an abnormally fast increase in pressure is detected within thehousing of an electrical device.

BACKGROUND

[0003] Electrical power distribution grids use electrical components,such as transformers, capacitors, and reactors. Potentially dangerousconditions can be created in such devices when aging or operatingstresses cause the insulation system to fail. A short circuit withinsuch a device can release a large amount of energy within a fraction ofa second. In the worst case the device can explode due to rapid internalpressure buildup from the vaporization of insulating oil and thedecomposition of the oil vapor into combustible or volatile gases.

[0004] Nearly all pole mounted distribution transformers are protectedby a cutout which includes an expulsion fuse or some other fast actingprotective device. Such cutouts can minimize damage by disconnecting afaulty device from its source of electric energy so as to interrupt arccurrent in the event of an overload or internal fault. Service personnelcan also use cutouts as manual switches for energizing or disconnectingparticular circuits. If there is an overload in the system and thecutout operates, then service personnel can easily spot the open cutoutand know that the transformer disconnected by the open cutout is out ofservice. If the fault is downstream of the transformer then, once thatfault has been corrected, it is a simple matter for service personnel tore-fuse the cutout to re-energize the circuit.

[0005] If the fault is in the transformer then closing the cutout beforethe transformer has been repaired will likely produce arcing within thetransformer. A device that has failed once is certain to fail again ifit is re-energized before the internal damage caused by the arcing hasbeen corrected. Arcing can leave carbonized paths within the device andmay impair the mechanical integrity of the device's housing, or “tank”.This increases the risk that the device will fail catastrophically if itis re-energized. In extreme cases the transformer may explode. Thiscould cause property damage and serious injury to service personnel andany members of the public who happen to be close by. To avoid thispossibility service personnel must perform careful inspections and takespecial precautionary measures before attempting to re-energize anyelectrical apparatus found disconnected from the power system by itsprotective device.

[0006] Unfortunately, an internal fault can occur in a device withoutleaving any obvious visible cues that the fault has occurred in thatdevice. Unless service personnel can tell that a particular device hasfailed they may reapply power to the device without detecting that theelectrical device has failed. This may cause catastrophic failure of theelectrical device, as noted above.

[0007] It is known that there is a transient surge in pressure insideoil-filled electrical devices, such as transformers, when the devicessuffer from an internal arcing fault. This happens because arcingproduces a marked increase in temperature which vaporizes some of theoil. Some electrical devices are filled with electrically insulatinggases such as SF₆. In such gas-filled devices arcing causes pressuresurges in the gas.

[0008] There exist fault detectors capable of providing a visualindication that a device has failed. Such fault detectors accelerate therestoration of services while minimizing the possibility that a faileddevice will be re energized as a result of a human error. U.S. Pat. No.5,078,078, invented by Cuk, who is also the inventor of this invention,describes a device for detecting transient surges in pressure within thehousing of a transformer or similar device. The device fits in anopening in a casing of the transformer. A moveable piston senses rapidpressure surges which result from internal arcing faults within thetransformer. The piston has at least one aperture in it so that slowincreases and decreases of pressure within the transformer do not causesignificant motion of the piston. An indicator attached to the pistonchanges appearance when the piston has moved a predetermined distanceand retains the indication until reset. A disadvantage of the Cuk deviceis that the change in appearance of the indicator may not be readilyapparent, especially from a distance. Furthermore, there is no easy wayto prevent false triggering during transport and installation.

[0009] U.S. Pat. No. 5,623,891 discloses another device for detectingtransient surges in pressure within the housing of a transformer. Thedevice has a diaphragm which is subjected to internal pressure surgeswithin the transformer. The diaphragm carries a trigger retainer whichengages a trigger on an indicator shaft mounted for rotation within thehousing. An indicator is mounted on the indicator shaft beneath a lenswhich is visible from the outside of the housing. A bias spring biasesthe indicator shaft toward rotation relative to the housing when thetrigger is engaged with the trigger retainer. When the diaphragm movesin response to a pressure surge in the transformer, the trigger retainermoves away to release the trigger, and the bias spring rotates theindicator shaft and indicator so that a warning section on the indicatoris positioned for viewing through the lens.

[0010] The device disclosed in U.S. Pat. No. 5,623,891 has a number ofdisadvantages including:

[0011] The inner surface of the indicator lens can become fouled byfumes generated during an internal fault;

[0012] The outer surface of the indicator lens can be obscured by ice orsnow;

[0013] The device provides no way to prevent false triggering duringtransport and installation;

[0014] It is typically necessary to mount the device on the top surfaceof a transformer, in a position where the lens is not readily visiblefrom the ground;

[0015] The device may be falsely triggered by oil splashing or the like;and,

[0016] If the device is to be mounted on the side of a transformerhousing then there must be a larger than usual air space in thetransformer housing or the diaphragm will be partially submerged.

[0017] There is a need for internal fault indicators for electricalequipment of the type used in electrical power distribution whichprovides a clear visual indication that a device has experienced a faultand which avoids some of the disadvantages of the prior art.

SUMMARY OF THE INVENTION

[0018] This invention provides a warning indicator for oil-filledelectrical equipment, such as power transformers or the like. Thewarning indicator detects pressure surges created in the housing of theelectrical device by an internal electrical fault and yet is insensitiveto pressure changes due to normal temperature variations. The indicationmay be used to positively identify an electrical device which hassuffered from an internal fault so that service personnel will be warnednot to undergo potentially hazardous attempts to re-energize a faultypiece of electrical equipment.

[0019] Accordingly, one aspect of the invention provides a faultindicator for indicating the occurrence of a rapid pressure surge withina housing of an electrical device. The fault indicator comprises: abarrel capable of being mounted in an opening in a housing of anelectrical device; and an actuating mechanism. The actuating mechanismcomprises: a chamber within the housing, the chamber having at least oneorifice communicating between interior and exterior surfaces of thechamber; and, an actuating member movable in responses to a pressuredifferential between the interior and exterior surfaces of the chamber.The fault indicator also has a plunger within a bore of the barrel. Theplunger is biassed outwardly in the barrel and is normally retained inan armed position by the actuating member. When the pressuredifferential is positive, the actuating member is moved and therebypermits the plunger to move outwardly in the bore to a triggeredposition.

[0020] Preferably the chamber comprises a diaphragm and the actuatingmember is attached to the diaphragm. The actuating member preferablycomprises a trigger pin which projects from the diaphragm and engages atrigger notch in the plunger when the plunger is in its armed position.The diaphragm and the barrel are both preferably oriented generallyhorizontally. This makes the fault indicator compact.

[0021] Another aspect of the invention provides a fault indicator forindicating the occurrence of a rapid pressure surge within a housing ofan electrical device. The fault indicator comprises: pressure surgedetecting means for moving an actuating member in response to a rapidrise in pressure within a housing of an electrical device; indicatormeans actuated by the pressure surge detecting means, the indicatormeans comprising a plunger movably disposed within a bore, the plungermovable outwardly in the bore from an armed position to a triggeredposition upon movement of the actuating member; and, retaining means forpreventing the plunger from becoming separated from the fault indicator.

[0022] A still further aspect of the invention provides a method forindicating the occurrence of a rapid pressure surge within a housing ofan electrical device. The method comprises: providing a chamber within ahousing of an electrical device, the chamber comprising an enclosedvolume and an orifice communicating between the enclosed volume and anair space within the housing and providing a plunger having a hiddenportion which is hidden from view; allowing a rapid pressure surgewithin the housing to displace a wall portion of the chamber inwardly;in response to motion of the wall portion releasing a plunger; and,moving the plunger so that the portion of the plunger which was hiddenfrom view is exposed.

[0023] Further features and advantages of the invention are describedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In figures which illustrate non-limiting embodiments of theinvention:

[0025]FIG. 1 is a partially cut away view of an electrical powertransformer equipped with an internal fault indicator according to theinvention and connected to an energy supply;

[0026]FIG. 2A is a section through an internal fault indicator accordingto the invention;

[0027]FIG. 2B is a detailed sectional view of a portion of the internalfault indicator of FIG. 2A with a shipping lock in place to preventpremature triggering of the internal fault indicator;

[0028]FIG. 2C is a section through an internal fault indicator accordingto an alternative embodiment of the invention wherein a coil spring isused to provide a bias force on a trigger pin;

[0029]FIG. 3 is an exploded view of the internal fault indicator of FIG.2;

[0030]FIG. 4 is a perspective view of the internal fault indicator ofFIG. 2 before activation;

[0031]FIG. 5 is a perspective view of the internal fault indicator ofFIG. 2 after activation;

[0032]FIG. 6 is a perspective view of the internal fault indicator withits shipping lock in place; and,

[0033]FIG. 7A is a schematic view showing one possible arrangement forpreventing the rotation of a barrel of an internal fault indicatoraccording to the invention in an aperture in a housing of an electricaldevice;

[0034]FIG. 7B is a schematic view showing another possible arrangementfor preventing the rotation of a barrel of an internal fault indicatoraccording to the invention in an aperture in a housing of an electricaldevice;

[0035]FIG. 8 is a sectional view of an internal fault indicatoraccording to an alternative embodiment of the invention;

[0036]FIG. 9 is an exploded view of the internal fault indicator of FIG.8;

[0037]FIG. 10 is a partially cut away view of the fault detector of FIG.8 with some parts removed for clarity;

[0038]FIGS. 11A and 11B are detailed partially cut away viewsillustrating a locking mechanism of the internal fault indicator of FIG.8 in locked and unlocked configurations respectively; and,

[0039]FIG. 12 is an isometric view of a diaphragm of alternative designthat may be used in fault detectors according to the invention.

DESCRIPTION

[0040] The invention will now be described using the example of aninternal fault indicator for a power transformer. It will be appreciatedthat the invention has application to high power electrical devicesgenerally and not just to transformers. FIG. 1 shows a typicaldistribution pole 10 with a crossarm 12 supporting power lines 14. Atransformer 16 is mounted on the pole 10 and is connected via a fusedcutout 18 to one of the lines 14. When the cutout opens, it hinges downas illustrated in dashed outline in FIG. 1. This breaks the circuitbetween transformer 16 and line 14.

[0041] Transformer 16 has a housing or “tank” 20. An internal faultindicator 22 (which could also be called an internal fault detector) ismounted in an aperture 24 (best shown in FIG. 7A) in a side wall ofhousing 20. Aperture 24 is preferably a small hole. Aperture 24 may, forexample, be a hole about 1 inch (25.4 mm) in diameter. Housing 20contains electrically insulating oil (or gas). Internal fault indicator22 is located in an air space above the level of oil in housing 20.

[0042] As shown in FIG. 2A, internal fault indicator 22 comprises anactuator mechanism, indicated generally by 26, which detects transientpressure surges within housing 20, and an indicator mechanism, indicatedgenerally by 28, which changes appearance when the actuator mechanismhas detected a transient pressure surge. Preferably internal faultindicator 22 also comprises a shipping lock 30 which, when installed,prevents indicator mechanism 28 from being triggered. Internal faultindicator 22 can also conveniently include an integral pressure reliefvalve 32. Shipping lock 30 may prevent pressure relief valve 32 frombeing actuated.

[0043] When there is a breakdown of the insulation surrounding theenergized or “active” components of transformer 16 an electric arc iscreated. The electric arc dissipates large amounts of energy. The suddendissipation of energy within housing 20 causes a sharp rise in thepressure within housing 20. Even at levels of short circuit current onthe order of 100 amperes, or less, the pressure within housing 20 risesat a rate which is distinctly higher then any other pressure rises thatare reasonably expected to occur in normal operation of transformer 16.This rapid pressure rise is detected by actuator mechanism 26 whichtriggers indicator mechanism 28.

[0044] If the pressure rises to a value which is greater than the setpoint of pressure relief valve 32 then pressure relief valve 32 opensuntil the pressure has been relieved. The pressure within housing 20 mayrise to a level capable of opening pressure relief valve 32 as a resultof normal fluctuations in ambient temperature and loading. Servicepersonnel may also manually operate pressure relief valve 32, asdescribed below, to equalize the ambient pressure inside housing 20 withthe air pressure outside of housing 20.

[0045] As shown best in FIGS. 2A and 3, actuator mechanism 26 comprisesa chamber 40 which is open to the interior of housing 20 only by way ofa small orifice 42. A thin diaphragm 44, which functions as a gasbarrier, supported by a compliant substructure 46 forms one wall ofchamber 40. In the illustrated embodiment the compliant substructurecomprises a spiral spring 48. Spiral spring 48 is preferably configuredto lie on a spherical surface when it is at equilibrium. Diaphragm 44has one face 44A in chamber 40 and a second face 44B exposed to theambient pressure within housing 20. Chamber 40 is preferably roughlysemi-spherical so that it can occupy a reasonably small space withinhousing 20. Diaphragm 44 preferably has a reasonably large area so thatpressure differentials across diaphragm 44 will generate sufficientforces to trigger indicator mechanism 28. Diaphragm 44 may, for example,have a diameter of 3 inches or more. For maximum reliability andsensitivity, diaphragm 44 should face downward toward the surface of theoil in housing 20.

[0046] Because air can enter or leave chamber 40 by way of orifice 42,the air pressure within chamber 40 will track slow changes in theambient pressure within housing 20. Such changes might occur, forexample, when the temperature of transformer 16 changes. On the otherhand, if the pressure within housing 20 increases very suddenly, the airpressure within chamber 40 will take some time to increase because ofthe small size of orifice 42. Preferably diaphragm 44 moves far enoughto reliably trigger indicator mechanism 28 in response to pressuresurges which are more rapid than about 0.5 to 1.5 psi over 5 ms anddiaphragm 44 is insensitive to fluctuations in the ambient pressurewithin housing 20 which occur more slowly than about 1 psi per second.During this period the pressure on face 44B of diaphragm 44 willtemporarily significantly exceed the pressure on face 44A. Diaphragm 44is pushed toward chamber 40. This would occur if an electrical fault inthe active components of transformer 16 caused an electrical arc withinhousing 20. A splash cover 49 dampens the effects of oil splashing ontodiaphragm 44 as might occur, for example, if housing 20 was shaken by anearthquake.

[0047] An axial guide rod 50 extending from spiral spring 48 projectsinto orifice 42. The location of the end of guide rod 50 projectingthrough orifice 42 can be used to verify that spiral spring 48 has beenproperly located within chamber 40 during assembly. The movement ofdiaphragm 44 triggers indicator mechanism 28. In the illustratedembodiment a trigger pin 54 projects from face 44B of diaphragm 44.Trigger pin 54 may be press fit into a hub located in the centralportion of spiral spring 48. Under normal operating conditions chamber40 is exposed to various mechanical vibrations and shocks includingseismic tremors. To avoid false triggering by such mechanicalvibrations, and to permit rapid operation, the mass of diaphragm 44 andspiral spring 48 should be small. Diaphragm 44 can comprise a thin layerof an air impermeable material such as 5 mil polyethylene film. Spiralspring 48 may be fabricated from a thin sheet of a suitably resilientplastic.

[0048] Indicator mechanism 28 comprises a barrel 56. A flanged outer end56A of barrel 56 projects through aperture 24. An all weather gasket 57is captured between outer end 56A and the outer surface of housing 20. Anut 58 threaded onto a threaded shoulder 56B on barrel 56 is tightenedagainst the interior wall surface of housing 20 to ensure the integrityof the seal around aperture 24. Barrel 56 should be prevented fromrotating in hole 24. This may be accomplished, for example, by makingaperture 24 D-shaped with a flat 59B in aperture 24 which engages acorresponding flat 59A on shoulder 56B (see FIG. 7A). FIG. 7B shows analternative construction which prevents rotation of barrel 56 relativeto aperture 24. In the embodiment of FIG. 7B, a projection 59D onhousing 20 engages a notch 59C in shoulder 56B.

[0049] Preferably barrel 56 is small enough to fit into an aperturewhich is approximately 1 inch in diameter. Barrel 56 is made ofnon-conductive material so that barrel 56 does not provide a conductivepath through the wall of housing 20. Barrel 56 may, for example, befabricated from fiber-reinforced polypropylene with additives to provideresistance to degradation by the action of sunlight.

[0050] A plunger 60 is located within a bore 56C of barrel 56. Plunger60 is urged outwardly by an eject spring 62 which is compressed betweenplunger 60 and an inwardly projecting flange 56D at an inner end 56E ofbarrel 56. Preferably eject spring 62 is received within a cylindricalextension 60D of plunger 60. Eject spring 62 is preferably attached bothto barrel 56 and to plunger 60. This may be accomplished by providingtails 62A on either end of eject spring 62. Tails 62A positivelyinterlock with mating features on plunger 60 and barrel 56.

[0051] Until internal fault indicator 22 is triggered, plunger 60 isprevented from being ejected from barrel 56 by the engagement of triggerpin 54 in a trigger notch 64 in plunger 60. Trigger pin 54 passes intobore 56C through a chamfered guide opening 65. Spiral spring 48 providesa slight spring force which tends to seat trigger pin 54 in triggernotch 64. Plunger 60 preferably has a flanged outer end 60B which bearsagainst a sealing ring 67 near the outer end of bore 56C. This sealsopening 24 while plunger 60 remains in its armed position within bore56C.

[0052] The side surface 60C of plunger 60 is brightly colored, andpreferably has a color which has high contrast to the colors typicallyfound in the environment of a transformer 16. Preferred colors are blazeorange, and bright yellow. It can be appreciated from the foregoingdescription that, upon a rapid pressure rise within housing 20,diaphragm 44 is displaced away from barrel 56. This pulls trigger pin 54out of trigger notch 64. Eject spring 62 then pushes plunger 60 out ofbore 56C of barrel 56. Plunger 60 is pushed at least far enough outwardin bore 56C that a rear end of plunger 60 is past the location oftrigger pin 54 and side surface 60C, which was previously hidden fromview within bore 56C is revealed, as shown in FIG. 5.

[0053] Preferably, after plunger 60 has been pushed outward in bore 56C,the outer end of plunger 60 extends significantly beyond the outeropening of barrel 56. This provides a highly visible indication that afault has occurred in transformer 16. The shape of internal faultindicator 22 is changed after plunger 60 has been ejected. Furthermore,after plunger 60 has been ejected its brightly colored outer surface 60Cis exposed to view.

[0054] Shortly after plunger 60 has been pushed outward in barrel 56 thepressure in chamber 40 will equalize with the ambient pressure withinhousing 20. This causes diaphragm 44 to resume its normal position. Whendiaphragm 44 has resumed its normal position, trigger pin 54 projectsinto bore 56C. Trigger pin 54 thereby blocks plunger 60 from beingpushed back into bore 56C. This prevents transformer 16 from being putunknowingly back into service without having passed an internalinspection. In general, whenever an electrical device has malfunctionedin a way that has triggered internal fault indicator 22, the deviceshould be opened and inspected before it is put back into service.

[0055] As best seen in FIG. 3, the illustrated embodiment of internalfault indicator 22 can be assembled by first affixing barrel 56 inaperture 24 as described above and then chamber 40 can be attached tobarrel 56. In the illustrated embodiment a combined chamber splash-guardassembly 68 has a groove 69 on its lower surface for receiving barrel56. The outer edges 69A of groove 69 are resilient and can be snappedover the outer surface of barrel 56. When barrel 56 is received ingroove 69, groove 69 engages and grips barrel 56. Withchamber/splashguard assembly 68 installed on barrel 56 (as shown in FIG.2A) trigger pin 54 passes into guide opening 65 on barrel 56.

[0056] Pressure relief valve 32 may be made integral with plunger 60.The pressure relief valve comprises an axially movable valve member 70which is biased into engagement with a valve seat 72 by a low ratespring 74. If the ambient pressure within housing 20 exceeds theatmospheric pressure outside of housing 20 then there is a net outwardforce on the end of valve member 70. When the this force exceeds apredetermined value, for example, a force corresponding to a pressuredifferential of 5 psi, spring 74 will compress and allow gases to ventfrom housing 20. Valve member 70 protrudes through a spring retainer 76to a vent cap 78. As valve member 70 moves axially outwardly, gases canescape from housing 20 by way of a venting gap between vent cap 78 andthe outer end 60B of plunger 60. A ring or other graspable member 79 maybe attached at the outer end of valve member 70 to permit manual ventingof housing 20. Combining an internal fault indicator and a pressurerelief valve in a single device avoids the need to provide two aperturesin housing 20 and conserves space within housing 20.

[0057] The outer end 56A of barrel 56 can receive a shipping lock 30.FIG. 6 shows a fault indicator 22 with a shipping lock 30 installed.Shipping lock 30 attaches to outer end 56A of barrel 56 and blocksplunger 60 from moving outward in bore 56C. Shipping lock 30 can be keptin place until after transformer 16 has been installed. Aftertransformer 16 has been installed, and before transformer 16 has beenput into service, shipping lock 30 is removed.

[0058] In the illustrated embodiment, shipping lock 30 comprises amember having a pair of inwardly directed flanges 82 which engagegrooves 84 (best seen in FIG. 2B) on outer end 56A of barrel 56. In theillustrated embodiment grooves 84 are defined between a stepped flangeon the end 56A of barrel 56 and the outer surface of housing 20.Preferably lock 30 must be broken to remove it from the end of barrel56.

[0059] Internal fault indicator 22 optionally includes a facility 85 forgenerating a control signal when the internal fault indicator istriggered. This facility may comprise one or more sets of electricalcontacts which close or open when internal fault indicator 22 istriggered. The electrical contacts may be operated to generate thecontrol signal, for example, by the passage of plunger 60 in bore 56C,or by the motion of trigger pin 54. The electrical contacts may be in afirst position (either closed or open) when plunger 60 is in its armedposition. As fault indicator 22 is triggered the electrical contacts areswitched so that when plunger 60 is in its triggered position thecontacts are in a second position (either open or closed). Facility 85may comprise other mechanisms such as fiber optics for communicating acontrol signal indicating to transmitter 86 that internal faultindicator 22 has been triggered. A transmitter 86 generates a faultsignal such as a radio signal in response to the control signal.

[0060] It can be appreciated that the internal fault indicator depictedin the accompanying figures has a number of advantages over prior artfault indicators. The ability to provide a single device which functionsboth as a pressure relief valve and as an internal fault indicatorprovides significant advantages over prior devices. It simplifies theconstruction of housings for electrical devices since a single openingin the housing can service both a fault indicator and a pressure reliefvalve. It also provides more latitude in arranging parts within theelectrical device housing. Space is at a premium inside the housing of atypical electrical device. This is especially the case in the topportion of the electrical device. A pressure relief valve and aninternal fault indicator should both be in the air space at the top ofthe housing. In typical electrical devices power leads also enter thehousing through the upper air space region.

[0061] Providing a plunger 60 which is expelled from bore 56C when faultindicator 22 is triggered results in a visual indication that a faulthas occurred in a device which can be seen much more clearly andunambiguously than has been previously possible. Both the apparent“shape” and color of the fault indicator change upon actuation. Further,because the fault indicator can be mounted in a side wall of a housing20, it can display its indication in a location which is more readilyvisible from the ground than previous fault indicators which aremountable only in the top surface of a housing.

[0062] The construction of internal fault indicator 22 which includes asensor diaphragm and an indicator element wherein, upon actuation, theindicator element moves in a direction generally parallel to a plane ofthe diaphragm provides a compact internal fault indicator 22 which canbe mounted in the air space at the top of housing 20 and yet has a largeenough diaphragm area to provide good sensitivity to pressure surgesinside housing 20.

[0063] Providing an indicator element which cannot be returned to itsinitial position after internal fault indicator 22 has been triggeredwithout opening housing 20 reduces the likelihood that, through humanerror, an electrical device will be placed back into use before it hasbeen properly inspected and serviced.

[0064] A diaphragm assembly which includes a spiral spring, for example,the spiral spring 48 shown in FIGS. 2A and 3 has the advantage that itis self-centering and allows easy axial movement of trigger pin 54.

[0065] Internal fault indicator 22 can be made so that it projects fromhousing 20 by only a minimal amount. Thus there are no surfaces to whichsnow and ice are likely to adhere.

[0066] Where internal fault indicator 22 will be used in electricalapparatus, the fault indicator should be designed and constructed toprovide longevity, and high reliability under all expected operatingconditions. Further, the components of internal fault indicator 22should, as much as possible be made from non-conductive materials so asto interfere as little as possible with the distribution of electricfields in the device.

[0067]FIGS. 8 through 11B illustrate features of an internal faultindicator 122 according to an alternative embodiment of the invention.Internal fault indicator 122 functions generally as described above.Parts of fault indicator 122 which provide similar functions to parts offault indicator 22 have been assigned reference numerals which areincremented by 100 in comparison to the corresponding reference numeralsused in FIGS. 1 through 7.

[0068] Fault indicator 122 can be mounted into a housing 120 byinserting threaded portion 156B through an aperture 124 in the housingand securing fault indicator 122 in place with a nut 158. This makesfault indicator 122 somewhat easier to mount than fault indicator 22.Fault indicator 122 may be locked for shipping by passing a member, suchas a cotter pin (not shown) through holes 183A and 183B at the outer endof barrel 156. The locking member blocks plunger 160 from movingoutwardly.

[0069] Another feature of fault indicator 122 is that pressure reliefvalve 132 is removable as a unit from plunger 160. Pressure relief valveassembly 132 is received in a bore 161 in plunger 160. Plunger 160 hasan aperture 161A on its lower side which connects to bore 161. Whenpressure relief valve assembly 132 is removed then bore 161 and aperture161A provide access to the interior of housing 120. This access may beused, for example, to introduce dielectric oil or other fluids intohousing 120, to sample or remove fluid from within housing 120 or thelike. If such access is provided by fault indicator 122 then it may bepossible to eliminate ports from housing 120 which would otherwise benecessary to provide access to the interior of housing 120.

[0070] Where fault indicator 122 provides access to housing 120 asdescribed above, it is generally desirable to provide a disablingmechanism which prevents fault indicator 122 from being triggered whileit is being used as an access port. In the illustrated embodiment thedisabling mechanism comprises a pin 190 which projects into bore 156C ofbarrel 156 and a hook 191 formed in plunger 160. After pressure reliefvalve 160 has been removed, fault indicator 122 can be disabled byrotating plunger 160 within bore 156 until hook 191 engages pin 190 asshown in FIG. 11A. When it is desired to enable the operation of faultindicator 122, plunger 160 can be rotated until hook 191 is completelydisengaged from pin 190 as shown in FIG. 11B and then pressure reliefvalve 132 can be locked in place in bore 161. In the illustratedembodiment, pressure relief valve 132 comprises a pair of resilient arms132A (FIG. 9) which engage recesses in bore 161 and thereby holdpressure relief valve assembly 132 in place in bore 161.

[0071] Fault indicator 122 has a diaphragm 144 comprising a sheet of alightweight stiff material, such as polyester, formed to provide anumber of concentric annular ridges 147. This diaphragm configurationhas been found to provide good sensitivity to the occurrence of faults.

[0072] As will be apparent to those skilled in the art in the light ofthe foregoing disclosure, many alterations and modifications arepossible in the practice of this invention without departing from thespirit or scope thereof. For example:

[0073] the single orifice 42 shown in the drawings could be replacedwith a number of smaller orifices or some other construction whichlimits the rate at which the pressure within chamber 40 can followfluctuations in the ambient pressure within housing 20;

[0074] the shape of orifice 42 may be annular, as illustrated on FIG. 2or some other shape;

[0075] compliant substructure 46 is preferably a spiral spring but isnot necessarily a spiral spring;

[0076] compliant substructure 46 could be integral with diaphragm 44;

[0077] in the illustrated embodiment trigger pin 54 prevents plunger 60from being reinserted into bore 56C after internal fault indicator 22has been triggered. A separate pawl or other one-way ratchet mechanismcould be provided so that internal fault indicator 22 can be reset onlyfrom inside housing 20;

[0078] While it is desirable that ejection spring 62 be attached to bothplunger 60 and barrel 56 a separate retainer cord could be provided toprevent plunger 60 from falling completely away from internal faultindicator 22 upon actuation. Ejection spring or a retainer cordconstitute “retaining means” which function to prevent plunger 60 frombecoming separated from fault indicator 22 by being attached to bothplunger 60 and transformer 16;

[0079] various mechanical linkages may be used to release plunger 60 inresponse to motion of diaphragm 44;

[0080] Instead of using diaphragm 44 or a compliant support member, suchas a spiral spring 48 to bias pin 54 toward plunger 60, a separate biasmeans, such as a spring 80 could be used to bias diaphragm 44 towardplunger 60 as shown in FIG. 2C. In FIG. 2C, a thin gas barrier 44′ issupported by a lightweight stiffener plate 45 from which pin 54projects. A coil spring 80 urges pin 54 into engagement with plunger 60;

[0081] Diaphragm 44 may take any of a number of different forms. Forexample, FIG. 12 shows an alternative diaphragm 244 comprising a sheetof a stiff, lightweight material having radially extending ribs 245formed integrally with it.

[0082] In place of a chamber 40 closed on one side by a flexiblediaphragm, actuator mechanism 26 could comprise:

[0083] A chamber closed by both a relatively high mass piston and arelatively low mass piston. The two pistons may be concentric with oneanother and are connected to springs having the same spring constant.The inertia of the large mass piston prevents the large mass piston frommoving in response to sudden pressure surges. The large mass piston andthe small mass piston can both move in response to slow pressurefluctuations. Relative motion of the large mass and small mass pistonscan be used to release indicator mechanism 28;

[0084] Chamber 40 may comprise the interior of a bellows having rigidend faces joined by a flexible cylindrical wall. Relative motion of therigid end faces can trigger indicator mechanism 28 by way of a suitablemechanical linkage. One or more openings in the bellows will prevent theend faces from moving in response to slow fluctuations in the ambientpressure within housing 20; In non-preferred embodiments of theinvention, diaphragm 44 could be replaced with a rigid or semi-rigidmovable piston which is displaced toward chamber 40 in response tosudden pressure surges within housing 20;

[0085] a chamber 40 closed on one side by a diaphragm, as describedabove, for example, or any of these alternative mechanisms constitute“pressure surge detecting means” which respond to surges in pressurewithin housing 20 by moving a portion of a wall of a cavity with a forcesufficient to operate an indicator mechanism 28;

[0086] plunger 60 may have a different shape from the shape describedabove, for example, plunger 60 could comprise a flag, rod, plate, or thelike having hidden portions which are hidden from view within bore 56Cwhen plunger 60 is in its armed position and are revealed when plunger60 moves to a triggered position. A plunger 60 as described above, andany of the alternatives described herein for displaying an indicationthat internal fault indicator has detected a fault, constitute“indicator means”;

[0087] The locking device could attach to housing 20 or device 22 in amanner different from that illustrated herein. The locking device couldbe a different kind of member which prevents plunger 60 fromaccidentally moving to its triggered position before internal faultindicator 22 is put into service. For example, the locking device couldcomprise a pin (not shown) which passes through an aperture in plunger60 and therefore prevents plunger 60 from moving longitudinally inbarrel 56 until the pin is removed. The locking device could alsocomprise, for example, a sliding or pivoting or break-away member at theouter end of plunger 60 which blocks plunger 60 from moving outwardly inbarrel 56.

[0088] Ejector spring 62 could comprise an extension spring arranged topull plunger 60 outward in bore 56C in place of the illustratedcompression spring.

[0089] A disabling mechanism could comprise a different mechanicalconstruction for preventing plunger 160 from moving outwardly whilefault indicator 122 is being used as an access port. For example, thedisabling mechanism could comprise a stop member which is movablebetween a locked position in which it blocks outward movement of plunger160 and an unlocked position in which the stop member does not preventmotion of plunger 160 when fault indicator 122 is triggered.

[0090] Accordingly, the scope of the invention is to be construed inaccordance with the substance defined by the following claims.

What is claimed is:
 1. A fault indicator for indicating the occurrenceof a rapid pressure surge within a housing of an electrical device, thefault indicator comprising: a) a barrel capable of being mounted in anopening in a housing of an electrical device; b) an actuating mechanismcomprising: i) a chamber within the housing, the chamber having at leastone orifice communicating between interior and exterior surfaces of thechamber; and, ii) an actuating member movable in responses to a pressuredifferential between the interior and exterior surfaces of the chamber;and, c) a plunger within a bore of the barrel, the plunger biasedoutwardly in the barrel and normally retained in an armed position bythe actuating member; wherein, when the pressure differential exceeds apositive threshold value, the actuating member is moved and therebypermits the plunger to move outwardly in the bore to a triggeredposition.
 2. The fault indicator of claim 1 wherein the chambercomprises a diaphragm and the actuating member is coupled to thediaphragm.
 3. The fault indicator of claim 2 wherein the actuatingmember comprises a trigger pin projecting from the diaphragm, thetrigger pin engaging a trigger notch in the plunger when the plunger isin the armed position.
 4. The fault indicator of claim 3 wherein thediaphragm and the barrel are both oriented generally horizontally. 5.The fault indicator of claim 4 wherein the diaphragm has a first facefacing the chamber and a second downward facing face and the faultindicator comprises an apertured splash guard covering the second faceof the diaphragm.
 6. The fault indicator of claim 4 comprising a springurging the plunger outwardly in the bore.
 7. The fault indicator ofclaim 6 wherein the spring extends into a cylindrical opening on aninner end of the plunger and wherein, upon triggering the spring canpush the plunger to a position wherein a rear end of the plunger isoutward in the bore from trigger pin.
 8. The fault indicator of claim 6wherein an exterior surface of the plunger is brightly colored.
 9. Thefault indicator of claim 1 comprising a spring urging the plungeroutwardly in the bore.
 10. The fault indicator of claim 9 wherein thespring is attached to both the plunger and the barrel.
 11. The faultindicator of claim 1 comprising a pressure relief valve within theplunger.
 12. The fault indicator of claim 11 wherein the pressure reliefvalve is removably mounted in a bore in the plunger and, when thepressure relief valve is removed, the bore provides access to aninterior of a housing in which the fault indicator is mounted.
 13. Thefault indicator of claim 12 comprising a disabling mechanism, thedisabling mechanism comprising a member projecting inwardly into thebarrel and a hook on the plunger that engages the member to prevent theplunger from moving outwardly in the barrel when the plunger is rotatedin the barrel in a first sense.
 14. The fault indicator of claim 2comprising a compliant substructure supporting the diaphragm.
 15. Thefault indicator of claim 14 wherein the compliant substructure comprisesa spiral spring.
 16. The fault indicator of claim 2 wherein thediaphragm comprises a thin sheet of a gas barrier material and the faultindicator comprises a spring urging actuating member into engagementwith the plunger.
 17. The fault indicator of claim 2 wherein thediaphragm comprises a sheet of a stiff material formed to provide anumber of concentric annular ridges.
 18. The fault indicator of claim 14comprising a guide pin projecting in an axial direction from a centralportion of the compliant substructure, the guide pin projecting at leastpart way into the orifice.
 19. The fault indicator of claim 1 comprisinga removable lock on an outer end of the barrel, the lock blockingoutward motion of the plunger in the bore.
 20. The fault indicator ofclaim 19 wherein the lock comprises a member which is broken uponremoving the lock from the outer end of the bore.
 21. The faultindicator of claim 19 wherein the lock comprises a member detachablymounted in an aperture at an outer end of the barrel.
 22. The faultindicator of claim 1 wherein an outer end of the plunger comprises aflange, the flange sealingly engaging a seal member on the barrel whenthe plunger is in its armed position.
 23. The fault indicator of claim 1comprising a one-way mechanism in the bore which prevents the plungerfrom being pushed from the triggered position back to the armedposition.
 24. The fault indicator of claim 1 comprising a signaltransmitter connected to broadcast a fault signal upon actuation of thefault indicator.
 25. The fault indicator of claim 1 comprising a set ofelectrical contacts, the electrical contacts being closed or open whenthe plunger is in its armed position and the electrical contactsswitching to open or closed when the plunger is in its triggeredposition.
 26. An electrical device having a closed housing, a faultindicator according to claim 1 with the chamber in the housing and thebarrel projecting through an aperture in the housing and one or moreactive electrical components within the housing.
 27. The electricaldevice of claim 26 wherein the aperture is in a side wall of thehousing.
 28. A fault indicator for indicating the occurrence of a rapidpressure surge within a housing of an electrical device, the faultindicator comprising: a) pressure surge detecting means for moving anactuating member in response to a rapid rise in pressure within ahousing of an electrical device; b) indicator means actuated by thepressure surge detecting means, the indicator means comprising a plungermovably disposed within a bore, the plunger movable outwardly in thebore from an armed position to a triggered position upon movement of theactuating member; and, c) retaining means for preventing the plungerfrom becoming separated from the fault indicator.
 29. A method forindicating the occurrence of a rapid pressure surge within a housing ofan electrical device, the method comprising: a) providing a chamberwithin a housing of an electrical device, the chamber comprising anenclosed volume and an orifice communicating between the enclosed volumeand an air space within the housing and providing a plunger having ahidden portion which is hidden from view; b) allowing a rapid pressuresurge within the housing to displace a wall portion of the chamberinwardly; c) in response to motion of the wall portion releasing aplunger; and, d) moving the plunger so that the portion of the plungerwhich was hidden from view is exposed.
 30. The method of claim 29wherein moving the plunger comprises pushing the plunger out of a borein the housing wherein the hidden portion of the plunger is a side faceof the plunger.